This invention is related to the invention disclosed and claimed in U.S. patent application Ser. No. 382,254 filed contemporaneously with this application.
The present invention pertains to color balancing of photographic printing paper.
Although the present invention has application in various photographic processes, it is particularly useful with producing positive color prints on appropriate color printing papers from both color negatives and color positive transparencies by the white-light method of exposure, using materials known as integral tripacks which have three superimposed light-sensitive emulsion layers.
Those skilled in the art will appreciate that it is rarely if ever possible to produce a print with accurate color rendition without adjusting for color balance variations which are caused by color-mix changes in lighting conditions and changes in the characteristics of films and printing papers. Color printing processes provide considerable latitude in making color correction in printing by selective variation of red, green, and blue-light exposures. There are two principal techniques for making such exposure variations in present use. In tricolor additive printing, three separate exposures of the negative or transparency to the printing paper are made respectively through red, green, and blue separation filters, wherein the three exposures are varied to control color correction. Exposure of the printing paper to varying amounts of red, green, and blue light, hereinafter referred to as the primary colors of light, is thereby achieved. In white-light substractive printing, a single exposure of the original negative or transparency to the printing paper is made through a combination of colored filters, wherein the filter densities and colors are varied to control color correction, thereby selectively filtering out varying amounts of the primary colors of light. In either case, color control is achieved by selectively varying the exposure of the primary colors of light to the printing paper, exposure being basically the product of light intensity and duration. Such colored light variations produce corresponding variations in the production of appropriately colored dyes by the three emulsion layers, the product depending upon whether negative to positive, or positive to positive printing is being performed.
Except for U.S. Pat. Nos. 4,166,701 and 4,257,702 most prior art color balancing techniques employ visual color judgments using test prints. Such tests may be of typical subjects, in which case, in the white-light subtractive technique, the use of color filters for viewing a test print to make a subjective judgment as to filtering changes for making subsequent prints is in widespread use. Another technique is to use a matrix of cyan, magenta, and yellow dyes in steps equivalent to some multiple of "color correcting units" (i.e. 5 cc or 10 cc) to produce a trial or test print by printing through the matrix and then to make a visual judgment of the processed print relative to some standard such as a standard gray card. This matrix technique suffers from some irremediable defects. For example, there is no precise correlation between the indicated degree of correction and the units of color correction found in the enlarger colorheads of diverse manufacture. With the development of and gain in popularity of enlarger colorheads employing dichroic filters to effect color balance control the disparity between the matrix and the enlarger has been accentuated since the dyed acetate materials, or dye transfer dyes employed in matrices, have light absorption characteristics significantly different from those exhibited by the dichroic interference filters. Furthermore, the matrices universally are designed to work in discontinuous stepwise fashion at intervals sufficiently large to make small but significant differences in color balance difficult or impossible to ascertain. Another basic disabling defect of most matrix systems is that they attempt to test for three independent variables in a two-dimensioned system, which is manifestly impossible to do.
The two above-cited patents and the accompanying Application Ser. No. 382,254 completely describe non-objective techniques for ascertaining the color balance of color printing paper. The three disclosed techniques, however, do require printing by the tricolor method and the use of a color analyzer. The present invention allows the use of white-light printing and does not require the use of a color analyzer. Due to contrast variations from one lot of color film to another of the same brand, the precision of color balance obtainable with the method of the present invention is somewhat less than that obtainable with the other three cited techniques. Nevertheless, the present invention does result in color balance determinations which are a great improvement over those obtainable with any other white-light printing system, and well within the limits needed to produce acceptable color prints for commercial as well as amateur production.
The eye, quite correctly, has been described as the most discriminating instrument for detecting color differences when two samples are presented to it side by side. Sadly, it lacks the equally important faculty for precisely judging just how much difference there is between side by side samples. Some other technique must be sought by which color differences may be quantitated. The most simple and accurate technique in color printing by which difference may be measured is the precise measurement of the exposure difference which produces a definite color change.
The present invention combines the color comparing faculty of the eye with the precision of photosensitometry to control the color balance of photographic color printing. The present invention also circumvents the difficulty posed by three variables in a two dimensional system by holding one color, red for example, constant while varying the other two continuously against each other and against the fixed color, thus entirely doing away with one basic fault of existing matrix-dye systems of color balancing. The use of a target-card and conventional color negative film without recourse to dye-structure matrices makes the test results completely compatible with the printing method, obviating another basic fault of dye-matrix systems. Whatever blunting of accuracy is found derives from the assumption of a median gamma for the photographic color film. The small degree by which any particular lot of color film will vary from the median gamma will introduce a correspondingly small error in color balance, which deviation normally will not be found to be diabling.